The present invention generally relates to a video disc player utilizing a flexible foil-type disc record made of synthetic material such as polyvinyl chloride, which disc record is rotated at its center at a high speed, for example, 1,800 rpm. and, simultaneously with the rotation of the disc record; hovers on a rotation-induced air cushion above a stationary support or table.
More particularly, the present invention pertains to a stylus grinding device for use in the video disc player of the type referred to above, which device is designed such as to grind the stylus of a pickup or transducer mechanism of the video disc player to an optimum shape under a predetermined grinding condition selected in accordance with the duration of actual service of the stylus.
It is well known that the stylus used in the pickup of the video disc player is made of a hard material such as a diamond. However, it has been found and well recognized that even the tip of the diamond stylus tends to wear as it engages under friction in the spiral groove on the recorded disc record while the latter is rotated at the high speed. On the other hand, it is also well known that the video disc record is a high density information storage medium, the pitch between the concave and convex of video signal of which being in the order of a few micrometers and, therefore, the wear of the stylus tip results in voltage reduction of an output signal from the transducer mechanism with consequent reduction in quality, such as drop-out, of the picture reproduced.
Hereinafter, a mechanism of wear of the pickup stylus used in the video disc player will be described in detail with particular reference to FIGS. 1(A) and (B) which aare respective views, on a substantially micrographically enlarged scale, of the pickup stylus engaging in one turn of the spiral groove on the disc record being shown in section as taken in a radial direction thereof and that as taken in a direction parallel to the direction of extension of the spiral groove on the disc record.
As best shown in FIG. 1(B), only a portion 1 of the pickup stylus 1, which is located at a trailing side with respect to the direction of rotation of the disc record 2 as indicated by the arrow, rides over several groove crests 2a simultaneously like a sleigh runner, which crests 2a are formed at the bottom of each turn of the spiral groove in the form of frequency-modulated video information. The hatched area indicated by A in FIG. 1(B) represents a portion of the surface on each side of the pickup stylus 1 which slidingly contacts a corresponding one of the opposed side walls 2c and 2b defining each turn of the spiral groove on the disc record 2.
As best shown in FIG. 1(A), as the duration of actual service of the pickup stylus 1 increases, the stylus tip gradually wears from the original shape, indicated by the real line, to a shape indicated by the broken line.
A cause of the wear of the stylus has been considered as a cumulative effect of carbonization, and consequent softening, of the stylus by the heat generated by the friction of the stylus 1 against the groove crests 2a and walls 2b and 2c, together with change in physical structure, and consequent deterioration, of the material used to form the pickup stylus, which occurs as the pickup stylus 1 is repeatedly used.
In order to ensure, even though the pickup stylus has worn to some extent, a good reproduction of the video information, recorded in the groove on the disc record, through a video display tube provided in the video disc player, or if not provided therein, a video display tube in a television receiver to which the video disc player is electrically coupled, a stylus grinding method has heretofore been proposed whereby the pickup stylus is subjected to a grinder each time one cycle of operation or scan of the pickup stylus relative to the disc record completes, that is, each time the pickup stylus has been used to reproduce the complete video information recorded on one disc record.
More specifically, as shown in FIGS. 2(A) and (B), according to the heretofore proposed method described above, upon completion of each cycle of scan of the pickup stylus relative to the disc record, the pickup carrying the stylus is moved radially outwards past the outermost one of turns of the spiral groove on the disc record with the stylus clearing of the disc record. A grinding disc 3, which is located outside the outer periphery of the disc record and which is rigidly mounted on a rotary disc 5 coupled to an electric motor 4, is applied to the pickup stylus under a predetermined contact pressure to grind said pickup stylus while said grinding disc 3 is rotated at a high speed, for example, 10,000 rpm.
The grinding disc 3 used in this example is formed by bonding fine particles of diamond, 1 micron in average particle size, over a support sheet 3a, for example, made of polyvinly chloride, and subsequently subjecting to a press work to form a plurality of concentric grooves 3b on the diamond bonded surface of the support sheet 3a, said grooves 3b being best shown in FIG. 2(C) which illustrates, on a substantially micrographically enlarged scale, of some of the concentric grooves 3b in section as viewed in a radial direction of the grinding disc 3. The resultant disc 3 is thereafter mounted on the rotary disc 5 for rotation together with said rotary disc 5.
In practice of the above described method, the pickup stylus 1 is held in contact with the grinding disc 3 under the predetermined contact pressure P while the stylus tip is, as best shown in FIG. 3, slidingly engaged in one of the concentric grooves 3b. Specifically, in the condition of FIG. 3, the opposed side areas at the tip of the pickup stylus 1 are held in sliding contact with respective side walls of the grinding groove 3b on the grinding disc 3 during rotation of the rotary disc 5, and, therefore, the grinding disc 3 in a direction as indicated by the arrow in FIG. 3. As can readily be understood, as the grinding operation proceeds for a substantially long period of time, the height h of the pickup stylus 1 decreases with consequent increase of the surface area A on each side of the pickup stylus 1. Although one cycle of grinding operation usually completes in a few seconds, reduction of the height h of the pickup stylus 1 together with consequent increase of the surface area A steadily occurs with repeated cycles of grinding operation. Increase of the surface area A on each side of the pickup stylus 1 results in reduction of the load received per unit surface area by each of the side areas of the pickup stylus 1 which are respectively in contact with the opposed walls of the grinding groove 3b during the grinding operation.
As regards the friction wear of the trailing portion of the stylus tip, the amount of wear occurring during each cycle of scan of the pickup stylus relative to the disc record can be considered to be constant irrespective of the surface area A which increases with repeated cycles of grinding operation subjected to the pickup stylus, since the contact area of the pickup stylus tip against the groove crests 2a is very small as shown in FIG. 1(B). Accordingly, the amount of the stylus tip to be ground, which is defined by d between the original level of the stylus tip and the ground level of the same stylus tip in FIG. 1(A), may be maintained constant irrespective of increase of the surface area A.
As can be understood from the foregoing discussion, the amount of stress per unit surface area which is applied from the grinding groove 3b to the pickup stylus 1, is smaller when the surface area A is relatively small, that is, at an initial stage of repeated cycles of grinding operation, than when the surface area A is relatively great, that is, at a later stage of the repeated cycles of grinding operation. Therefore, if the pickup stylus is ground under the same condition each time one cycle of scan of the stylus relative to the disc record without the increase of the surface area A being taken into consideration, the amount of the stylus tip to be ground becomes smaller at the later stage of the repeated cycles of grinding operation than at the initial stage of the repeated cycles of grinding operation.
In order to maintain the amount of the pickup stylus to be ground to be constant irrespective of the duration of actual service of the pickup stylus, the grinding condition to which the pickup stylus is subjected should be varied with increase in the duration of actual service of the pickup stylus.